feat(setup): query Base USDC balance + show OKX fund walkthrough

After eden's "show what chain to fund + how much" request. The PR #8
wallet-ready box hinted at funding but had no live balance check and
no concrete OKX-side steps. Users still asked "do I actually need to
fund?" and "where on OKX do I send from?".

Changes:

- Query Base-chain USDC balance via the existing BalanceMonitor right
  after OKX login completes (or right after detecting an existing
  session in the already-signed-in branch). Network/RPC failure is
  non-fatal — we render "could not query Base RPC" instead of aborting
  setup.

- Add a balance row to the wallet-ready summary box, severity-tagged:
    empty  (< $0.0001)   →  yellow ⚠ "fund to unlock paid models"
    low    ($0.0001..$1) →  yellow ⚠ "recommend $5+"
    ok     (≥ $1)        →  green ✓  "ready for paid models"

- Only render the funding walkthrough when balance is missing/empty/low.
  Already-funded users see a tight 6-line box; users that need money
  see the OKX-specific 3-step guide:

    Fund via OKX (Base network, USDC):
      1. Open the OKX app → Wallet
      2. Send USDC on Base to the address above
      3. ~$5 covers thousands of paid requests

  Naming OKX explicitly + naming Base explicitly avoids users bridging
  to a wrong network (the recurring support footgun).

- Lazy-import BalanceMonitor so the CLI doesn't pull viem until setup
  actually needs to query a balance.

Author: KillerQueen-Z